1. Field of the Invention
This invention relates to a system and method for reducing the probability of deadlock conditions caused by the repeated transmission of data frames containing data sequences equivalent to those used by serial data communications equipment for inter-device signalling.
2. Description of the Prior Art
In present use are several types of serial data communications devices (modems, multiplexers, encryptors) which are, in their simplest form, completely insensitive or transparent to the contents of a data stream. They employ special signaling patterns to communicate information from one device to an equivalent device at the other end of the communication plane. Such information might include, for example, pseudo-controlled-carrier operation in which the "carrier detect" output at one end of the link follows he "request-to-send" input at the other end without actual loss of carrier signal. Since these devices have no control over the data being sent through the communications link by the Data Terminal Equipment (DTE) it is always possible that one of the signaling patterns may occur by chance in the data stream. The response of the receiving device to the special sequence would generally result in loss or corruption of some portion of the link data. The typical approach taken to minimizing the likelihood of these losses is to use very long sequences of pseudorandom data or sequences which are known to be improbable based on the protocol(s) expected to be in use.
A more serious problem which occurs in the aforementioned situation than the initial loss of data is that the error recovery procedures of the higher level protocol in use on the communications link will call for a re-transmission of the lost data. If the initial (falsely detected) signalling pattern was completely contained in a single data frame, it will be transmitted by the DTE again and trigger the same erroneous response and loss of data at the receiver. This results in a deadlock condition which can probably only be removed by abandoning the attempts to send this particular frame of data. Thus the link is, in effect, opaque to the data i this frame.